1. Field of the Invention
The present invention relates to oxidized charge transport materials which can form stable hole injection layers in organic light emitting devices without utilizing a polymeric binder.
2. Description of the Related Art
Organic light emitting devices (OLEDs) typically comprise a layer of emissive material between an anode and a cathode. When a bias is applied across the electrodes, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the emissive layer. The holes and the electrons form excitons in the emissive layer to emit light.
Electrodes are chosen to facilitate charge injection. An indium-tin-oxide (ITO) anode has a relatively high work function and is therefore suitable for use as a hole injection electrode, while low work function metals such as Al, Mg and Ca are suitable for injection of electrons.
To improve the power efficiency of an OLED, it is frequently desirable to enhance charge injection at the electrode interface. Recently it has been found that hole injection at the anode interface can be facilitated by using an oxidized charge transport (OCT) polymer as a hole injection layer in an OLED.
Addition of an oxidant to a charge transport polymer (such as polyvinyl carbazole) to give a semiconducting charge transfer polymer was first demonstrated in the 1960s. Thin films of these materials have been referred to as xe2x80x9coxidized transport layersxe2x80x9d (OTLs). Conventional OTLs typically comprise a three-component coating: a charge transport molecule, an oxidized charge transport molecule, and a polymer binder. Different oxidant and charge transport molecule/polymer combinations have been developed and investigated for a wide range of applications. For example, U.S. Pat. No. 5,853,906, which is incorporated by reference herein, discloses conductive polymer compositions comprising an oxidized oligomer salt (including oxidized arylamines), a charge transport component and a polymeric binder, primarily for use in electrophotographic applications. Recently, OTLs have been investigated as a contact modification layer on the anode in OLED applications.
Organic EL devices with a hole injection layer formed from N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine (TPD) doped with oxidizing reagents such as I2, FeCl3, SbCl5 and tris(4-bromophenyl)aminium hexachloroantimonate (TBAHA) have also been disclosed.
A drawback of the known OTL systems is that they are unstable and cannot readily be incorporated into a device without utilizing a polymeric binder.
The most stable OTL known today is based on the cation radical salts of N,N,Nxe2x80x2,Nxe2x80x2-tetra-p-tolyl-4,4xe2x80x2-biphenyldiamine (TM-TPD.SbF6) having the structure: 
There continues to be a need in the art for hole injection materials exhibiting enhanced stability. There is a need in the art for OTLs fabricated with a variety of it triarylamine charge carriers. There is also a continued need for OTLs which can be formulated without a polymeric binder to form thicker and more robust hole injection layers which can be deposited by solvent processes such as spin coating.
In one aspect, the invention is an oxidized transport material formed substantially without a polymeric binder suitable for use as a hole injection layer in an OLED, comprising a charge transport compound having more than two triarylamine It groups, or having at least one triarylamine group and at least one fluorene group; and an oxidant complexed with said charge transport compound, wherein a portion of said charge transport compound is not complexed to the oxidant.
Advantageously, oxidant is present in the oxidized charge transport material in an amount between 0.2 and 20 percent by weight.
The oxidized transport material comprising the specified charge transport compounds is preferably fabricated as a thin layer in an organic light emitting device by solution processing, such as spin coating, without using a polymeric binder. Such layer generally has a thickness of about 50 Angstroms to about 50,000 Angstroms, preferably about 100 Angstroms to about 2,000 Angstroms. It is believed that the high molecular weight of the oligomeric charge transport compounds according to the invention provides these compounds with good film forming properties such that no polymeric binder is needed.
In another aspect, the invention is an organic light emitting device comprising an anode and a cathode with an emissive layer interposed between them, and between the anode and the emissive layer, a hole injection layer consisting essentially of charge transport compound having more than two triarylamine groups, or having at least one triarylamine group and at least one fluorene group; complexed with an oxidant, wherein a portion of said charge transport compound is not complexed with said
This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiment thereof in connection with the attached drawings.